This invention relates to the preparation of thermoplastic materials comprising homopolymers or copolymers of vinyl chloride and polyacrylate polymers as toughening agents.
Processes, according to the prior art, for the preparation of vinyl chloride graft copolymers in suspension on elastomeric polymer particles prepared in emulsion still have some disadvantages.
The preparation of graft polymers in which vinyl chloride is polymerized on polyalkyl acrylates--which serve as toughening agents is known. German Patents 1,082,734, 1,090,856 and 1,090,857 describe processes in which graft polymers containing about 2% to 25% by weight of acrylate rubber are obtained. However, the preparation of such graft polymers results in deposits on the reactor walls which can only be removed with difficulty. The particle size distribution (measured using an air-jet sieve) has a fine fraction below 40 .mu.m and a coarse fraction above 250 .mu.m. The fine fraction leads to turbidity and hence to pollution of the wastewater when the suspension polymer is separated from the aqueous phase. Moreover, dust problems occur during handling of the dry powder. The coarse fractions impair the surface quality of shaped products produced on modern high-speed processing machines.
German Offenlegungsschrift 2,162,615 discloses the preparation of graft polymers of vinyl chloride on polymers of the acrylate type, the final polymers containing 30% to 60% by weight of acrylate units. The acrylates are polymerized in the presence of small amounts of a polyfunctional ethylenically unsaturated monomer in emulsion. An emulsifier having a low HLB value (2 to 12) or a high one (&gt;40), such as, for example, the sodium salt of bis-(tridecyl) sulfosuccinate (HLB=4 to 7) or an alkylsulfate salt having a short alkyl chain, such as sodium 2-ethylhexylsulfate (HLB about 50), should be used for the emulsion polymerization. Vinyl chloride, a suitable suspension agent and an initiator soluble in the monomer are added to the aqueous emulsion of the rubber particles. The pH should be adjusted from 3 to 9. To prevent the formation of wall deposits, a highly viscous methylhydroxypropylcellulose is preferably used. The vinyl chloride graft polymerization is carried out in suspension.
The disadvantage of this process involves the use of the suspension agent which has a high molecular weight and is therefore difficult to handle. Furthermore, the bulk density of the graft polymers is undesirably low for conventional processing methods. In addition, when a mixture is formed with polyvinyl chloride, the resultant extruded products have a comparatively low impact strength and a poor surface on the one hand and a relatively low output rate is obtained for the extrusion process on the other hand.
European Offenlegungsschrift 0,313,507 discloses a process in which there is obtained a polymer having 70% to 40% by weight of polymerized units of vinyl chloride and 30% to 60% by weight of polymerized units of at least one acrylate and optionally other monomers copolymerizable with acrylates. In a first step, in an aqueous emulsion, acrylates and optionally other monomers are reacted with the addition of a monomer having at least two ethylenically unsaturated, nonconjugated double bonds, in the presence of an initiator and of a water-soluble salt of a fatty acid containing 12.degree. to 18.degree. C. atoms as an emulsifier, at an initial pH above 9, to yield a polymer having a glass transition temperature of less than 0.degree. C. In addition to the fatty acid salt, polymerization is carried out in the presence of at least one alkali metal or ammonium salt of an alkylsulfonic acid having 8.degree. to 20.degree. C. atoms and/or of an alkyl acryloylsulfonate having 3.degree. to 16.degree. C. atoms in the alkyl radical.
In a second step, water, a suspension agent, a precipitating agent for the emulsifiers used in the emulsion polymerization, an initiator, optionally further auxiliaries and vinyl chloride are introduced into the polymerization vessel and the aqueous polyacrylate dispersion is added. The precipitating agents are salts of polyvalent metal ions and/or acids, for example, hydrochloric acid, sulfuric acid, nitric acid, citric acid, formic acid or acetic acid are described in EP-OS 0 313 507 as being suitable as precipitating agents for the emulsifiers used in the emulsion polymerization of the acrylic acid alkyl ester. Water soluble alkaline earth or aluminum salts, such as, e.g., magnesium sulfate, calcium chloride, calcium acetate, barium chloride, barium nitrate, aluminum chloride and potassium aluminum sulfate are preferred.
The addition of a second emulsifier and precipitating agent presents problems during normal operation. Furthermore, this necessitates greater expense with regard to storage and filling.